Optical fibers have, for a variety of reasons, become the communication medium of choice, and, consequently, the techniques for fabricating optical fibers have undergone steady improvement and refinement. Improved techniques have, in turn, led to, or made possible, improvements in fiber design, thus the two aspects of optical fiber fabrication, i.e., design and fabrication, "feed" upon each other, to the ultimate benefit of the communications field.
An optical fiber typically comprises a central core having an index of refraction greater than that of the material surrounding the core, i.e., the cladding, which is typically a doped silica glass. The indices of refraction can be varied by appropriate doping to achieve the best or most desirable transmission characteristics. The fiber is generally drawn from a preform, a glass body, typically a rod, produced by, for example, a chemical vapor deposition on the interior of a heated glass tube or substrate. The heat causes a glass forming reaction to take place on the inner surface of the tube, and then heat is applied to collapse the glass tube, thereby forming a glass rod. Alternatively, the rod may be produced by an outside vapor deposition on a glass substrate, and then a silica sleeve is shrunk around the rod to form a preform rod ready for drawing. The present application deals primarily with the first mentioned, or interior, deposition process, although it is applicable to preform rods in general, regardless of the process by which they are formed.
More fiber can be drawn from the preform if the rod has a relatively large diameter, which can be achieved by the substrate tube being made thicker. However, in the inside deposition process, such a thick tube or substrate would have too great a thermal impedance, making it difficult to sustain the glass forming reaction inside the tube and subsequently to collapse the tube with conventional exterior heating sources. In order to overcome this problem, it has been proposed, and subsequently practiced, that an appropriately scaled-up amount of deposited glass be synthesized inside of a standard silica tube and the tube then be collapsed to form the rod-like preform. The rod is then overclad by a silica tube which is shrunk around the rod, thereby encasing it in an overcladding. The fiber is then drawn from the overclad preform, and has a core of diameter d, a first cladding region of diameter D.sub.1, a substrate tube derived cladding having a diameter D', and an overcladding of diameter D.sub.0. Control of the diameters and the different diametric ratios enables production of more optical fibers of the desired transmission characteristics from an overclad preform of a given length. U.S. Pat. No. 5,044,724 of Glodis et al., the disclosure of which is incorporated by reference herein, gives a detailed description of the foregoing process, as well as the various diameters and their relationships and the indices of refraction of the components of the completed fiber.
Overcladding thus achieves the desired goal of more fiber produced per preform. However, the use of a glass tube into which the rod-like preform is inserted presents problems unique to the process. For example, the overcladding tube may be slightly non-circular or eccentric, or it may have a bow such that the circular straight interior is less than that of a perfectly straight, unbowed tube, which places a limit on the diameter of the preform that can be inserted into the overcladding tube. In some cases the bow, for example, may be such that a preform cannot be inserted therein. Other possible geometric variations in the overcladding tube, such as wall thickness variations and/or taper of the internal diameter also place limitations upon the straight through internal clearance (STIC) of the tube, and, as a consequence, upon the dimensions (primarily outside diameter) of the preform rod that can be inserted in the tube.
It can readily be appreciated that a process and apparatus which makes possible the obtainability of precise and accurate measurements of the various parameters of a glass tube, such as outside diameter, inside diameter, bow, and taper would have a salutary effect on the manufacture of glass tubes as well as the manufacture of optical fibers. Such measurements should indicate whether a given tube can be used without further operation thereon, whether the tube can be straightened to make it viable, or whether it must be scrapped. In addition, information obtained from the measurements could indicate necessary changes in the process of making the tubes to increase usability and productivity.